Other Expenses - Schedule of General and Administrative Expenses (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Professional services	$ 1,177	$ 1,022	$ 3,616	$ 3,383
Operating and other costs	810	687	2,059	2,026
Subtotal	1,987	1,709	5,675	5,409
Non-cash compensation expenses
Restricted class A common stock earned	7,629	6,609	22,901	20,113
Director stock-based compensation	125	125	375	375
Subtotal	7,754	6,734	23,276	20,488
Total general and administrative expenses	$ 9,741	$ 8,443	$ 28,951	$ 25,897